Lebanese and Syrian activists chant pro- revolution slogans during a protest against the participation of Hezbollah in the Syrian war at Martyrs Square in Beirut on June 9. / Bilal Hussein, AP

by Mona Alami, Special for USA TODAY

by Mona Alami, Special for USA TODAY

BEIRUT - The decision by the United States to aid Syrian rebels militarily will bring it in direct conflict with Hezbollah, the designated terror group and paramilitary force that fights for the Syrian regime of Bashar Assad.

Hezbollah's involvement in Syria's civil war is aggravating a centuries-old conflict between the two major sects of Islam - Sunnis and Shiites.

President Obama has said his goal is to help the rebels protect themselves from Assad's military. Doing so will place the United States firmly on the Sunni side of the Middle East conflict and in opposition to the growing Shiite military axis of Iran-Syria-Hezbollah, experts say.

The Muslim divide is apparent in Lebanon, where Palestinian refugees in the tens of thousands live in a country controlled by Shiite Hezbollah, an anti-American terrorist group with thousands of fighters that has fought two wars with Israel.

There have been skirmishes and threats between the Palestinians and Hezbollah but no major combat. Should that change, the United States may find itself in the middle of a major war.

"Both sides know how fragile the situation is, and all parties are aware and concerned that the situation could violently explode in an instant," says Avi Melamed, a former Israeli senior official on Arab Affairs.

"Should a broad Sunni-Shiite collision break out in Lebanon, the major Palestinian organizations will not be able to continue to sit on the fence for very long," he says. "At some point - even if against their will - they will have to take a side - and they're not likely to choose Hezbollah."

Lebanese Sunnis have experienced a growing feeling of marginalization. It started with the assassination of Sunni Prime Minister Rafik Hariri in 2005, in which four members of Shiite group Hezbollah have been indicted.

Many Sunnis blame Hezbollah for the more recent assassinations of major Sunni figures Wissam Eid, a police investigator, and Internal Security Forces Gen. Wissam al-Hassan.

Sunnis resent Hezbollah's control of Lebanon's security apparatus, and the Sunni-led government ordered the party to shut down its private telecommunication network and to fire its airport security chief Wafiq Shouqair in 2008, which led to street clashes between Hezbollah members and Sunni fighters.

Sunni anger toward Hezbollah worsened with the party's decision to fight alongside the Assad regime in Syria against the mostly Sunni rebels.

"As far as Sunnis are concerned, Syria is occupied by Shiites, with Iran openly involved in the conflict," Melamed said.

In May, three rockets were fired on Dahieh, a Hezbollah stronghold on the outskirts of Beirut. Melamed and others said Palestinian factions may have been involved. Angry Palestinian refugees in the Ain al-Hilweh refugee camp in a formal protest burned the food, commodities and aid packages Hezbollah had distributed to them.

In the past few weeks, 40 people were killed in clashes in Tripoli in which Sunnis fought Alawites, members of a Shiite offshoot aligned with the Syrian regime. Assad is an Alawite.

Further south in the city of Sidon, Sunnis clashed with Shiites participating in a funeral procession, preventing them from burying a Hezbollah militant killed while fighting in Syria. Two Sunni sheiks with ties to Hezbollah were shot at in Sidon and in the Beqaa Valley by unknown assailants.

The disenchantment of Sunnis in Lebanon has led to the radicalization of some members of the community, particularly in areas where Sunni and Shiite neighborhoods border each other, such as Tripoli, the Beqaa Valley, Sidon and certain neighborhoods of Beirut.

"About 80 Tripoli Sunnis have gone to fight with the rebels in Qussayr in recent weeks," said Salafist Sheik Abu Ahmad.

They were responding to the call for jihad in Syria by Sheik Salem Rafei two months ago, he said. The son of Dai Islam al-Chahhal, the highest Salafist authority in Lebanon, also fought in Qussayr alongside the rebels.

In Palestinian enclaves around Lebanon, six new radical groups have beefed up their operations. Composed of former members of radical organizations such as Jund al-Sham, Fateh al-Islam and Osbat al-Ansar, they have been accused of coordinating with the Syrian Islamist rebel group Jabhat al-Nusra, which the US government has labeled a terror organization.

As the war in Syria rages on, Melamed warns of the dangers of increasing tensions between Lebanese Sunnis who are against the regime and Shiites who are with it.

"Tensions are rising by the minute in Lebanon. While the scope of Salafist jihadists is still limited and does not allow them to seriously challenge Hezbollah, there is an accumulation of reports of possible involvement of Jabhat al-Nusra in Lebanon that are extremely worrisome. It is reasonable to expect that a war in Syria will percolate in Lebanon," he said.